Scan chains have been widely used in digital circuits as a design-for-test (DFT) technique. Scan elements and clocking circuitry occupy about 30% of silicon area of an IC chip. It has been estimated that 10-30% of defects found in defective IC chips are associated with scan chains and that 50% of chip failures are caused by scan chain defects. Hence, diagnosing scan chain faults with good resolution is important to guide silicon debug, physical failure analysis (PFA), and yield learning process.
Chain fault diagnosis techniques can be classified into three categories: tester-based, hardware-based, and software-based techniques. In the tester-based diagnosis techniques, a tester, cooperated with a PFA equipment, is used to locate faulty scan cells. These techniques can provide a good chain diagnostic resolution based on, e.g., measuring signal change frequency. They may not be suitable, however, for volume diagnosis due to their high cost and long diagnostic time.
The hardware-based diagnosis techniques employ dedicated scan cell structures or scan chain designs. For example, multiplexes can be added to propagate data from one scan chain to another scan chain. Some global signals and specific circuitries such as XOR gates may also be added to enable certain scan cells to be either set or reset after shift-in, and faulty scan cells can then be identified by analyzing the shift-out bits. While these hardware-based techniques can effectively identify faulty scan cells, the extra diagnostic signals and circuitry could lead to prohibitive hardware overhead.
The software-based diagnosis techniques are based on algorithmic diagnosis processes. To determine candidates of faulty scan cells, observed failing and passing responses are analyzed. Without the requirement of being tied to an expensive tester or the need of dedicated hardware, these techniques are suitable for volume diagnosis. However, the diagnostic resolution may be problematic if scan chains are not carefully designed. It is thus desirable to develop techniques for scan chain stitching based on diagnosis needs.